Is this a thing in Pendragon? I know such an arrangement existed, whereby anywhere from half to a fortieth of a knights fee could be held in vassalage by someone, usually a house knight or a yeoman, which was payed for with a corresponding fraction of the Servitum Debitum. So for example since the Standard SD is 5.5, a half fief knight would simply pay his lord 2.75L or supply him that amount's worth of soldiers. Or simply be retained as a house knight, with the value of the land deducted from the stipend maybe? I'm a little fuzzy on how exactly it works, but certainly Domesday makes reference to half-fees and a novel I'm reading at the moment has a character serving as a knight with less than a whole fief. Not sure if this exists in Pendragon.

Yes, it does
although not stated explicitly
Book of Estate or Warlords makes room for fractional holdings

So could someone run me through the way a knight with a half fief might live?

So could someone run me through the way a knight with a half fief might live?

Very frugally?

Or he would be very dependent on other sources of income; borrowing from relatives, gifts from lords, tournaments (if in a late enough period), raiding/looting enemies, or possibly being a robber knight on the sly. And maybe possibly eventually getting something in place that improves the value of the fief; an improvement, being granted some sort of toll or other right, etc.

Very frugally?

Or he would be very dependent on other sources of income; borrowing from relatives, gifts from lords, tournaments (if in a late enough period), raiding/looting enemies, or possibly being a robber knight on the sly. And maybe possibly eventually getting something in place that improves the value of the fief; an improvement, being granted some sort of toll or other right, etc.

I agree with Oaktree--a knight would need another source of income.
But mostly, a knight simply could not live on half of a manor.
It would be held by someone else--a esquire or a messenger, or as an adjunct to a full manor.

Would it be conceivable to be a house knight, those four pounds upkeep supplied by his lord directly in return for serving in his household nearly year round, while at the same time possessing a 5L estate which he pays a fixed rent for or supplies a spear man or two, The render of which he uses to provide for a family?

Would it be conceivable to be a house knight, those four pounds upkeep supplied by his lord directly in return for serving in his household nearly year round, while at the same time possessing a 5L estate which he pays a fixed rent for or supplies a spear man or two, The render of which he uses to provide for a family?

In principle, if the liege wants to do that, why not? However, it would be much easier from the liege's perspective to just retain the wife as a lady-in-waiting for the liege's wife, and pay the extra 1 librum for the children as a bonus for the household knight's distinguished service.

5L estate would have SD of 2.75 librum, which rounds up nicely to 3L = a cavalryman and two foot soldiers. This would leave 2L for the wife and children, but nothing left over for a lady's maid nor the chaplain. Although if the liege is feeling generous, SD of 2.5L would mean a cavalryman and a foot soldier, with 0.5L for a lady's maid. There is also the fact that the render would only be 5L, too, which means that the servant staff would be undermanned, too, in comparison to a full manor.

Like Greg said earlier, those half-fees would be better as additional lands to the main manor, or perhaps a couple of slices that together make up for a full manor (parts of which had been lost to other families via split inheritance between female claimants).